Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. The oxygen can be either a pure form (O2) or air (a mixture of O2 and N2). PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive, solid polymer electrolyte membrane having the anode catalyst on one face and the cathode catalyst on the opposite face.
The MEA is sandwiched between a pair of non-porous, electrically conductive elements or plates which (1) pass electrons from the anode of one fuel cell to the cathode of the adjacent cell of a fuel cell stack, (2) contain appropriate channels and/or openings formed therein for distributing the fuel cell's gaseous reactants over the surfaces of the respective anode and cathode catalysts; and (3) contain appropriate channels and/or openings formed therein for distributing appropriate coolant throughout the fuel cell stack in order to maintain temperature.
The electrically conductive plates sandwiching the MEAs may contain an array of grooves in the faces thereof that define a reactant flow field for distributing the fuel cell's gaseous reactants (i.e., hydrogen and oxygen in the form of air) over the surfaces of the respective cathode and anode. These reactant flow fields generally include a plurality of lands that define a plurality of flow channels therebetween through which the gaseous reactants flow from a supply header at one end of the flow channels to an exhaust header at the opposite end of the flow channels.
The term “fuel cell” is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are typically bundled together to form a fuel cell stack and are commonly arranged in electrical series. Each cell within the stack includes the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. By way of example, some typical arrangements for multiple cells in a stack are shown and described in U.S. Pat. No. 5,763,113.
In a fuel cell stack, a plurality of cells are stacked together in electrical series while being separated by a gas impermeable, electrically conductive bipolar plate. In some instances, the bipolar plate is an assembly formed by securing a pair of thin metal sheets having reactant flow fields formed on their external face surfaces. Typically, an internal coolant flow field is provided between the metal plates of the bipolar plate assembly. It is also known to locate a spacer plate between the metal plates to optimize the heat transfer characteristics for improved fuel cell cooling. Various examples of a bipolar plate assembly of the type used in PEM fuel cells are shown and described in commonly-owned U.S. Pat. No. 5,776,624, issued Jul. 17, 1998 and hereby incorporated herein by reference. The internal coolant flow field consumes space within the fuel cell, typically increasing the required distance between adjacent MEA's.